How can a medium-sized model like Deepseek-V4-Flash be cheaper than a much smaller models like Qwen3.5-35B-A3B.

It's five times bigger in both total and active parameters!

I don’t know for sure, but I believe those larger models must be run on nVidia hardware (CUDA), while Deepseek-V4-* can be run on Huawei chips. My assumption is that there is less demand pressure on non-nVidia chips.